Exercise and Bone StrengthMariana Shedden, M.S and Len Kravitz, Ph.D.
IntroductionExercise has been shown to increase bone strength, as measured by bone mineral density (BMD), in people of all age groups. The degree to which bone strength improves, however, is dependent on a variety of factors, including age, reproductive hormone status, nutritional status, and the nature of the exercise. Since 50% of spine and 90% of hip fractures are attributable to falls, preventative exercise interventions should try to 1) minimize bone loss in adulthood, 2) improve peak bone mass, and 3) reduce the risk of falling. A complete understanding of how bone loading results in gains in BMD has not been entirely explained, and little data exist for men due to the focus on osteoporosis in women. This article will highlight contemporary concepts reviewed by
Beck and Snow (2003) on exercise for the improvement of BMD as it relates primarily to women throughout their lifespan.

EXERCISE DESIGNAccording to Drinkwater (1994), the following five training principles should be taken into account when designing an exercise program to increase bone mass: specificity, overload, reversibility, initial values, and diminishing returns.

SpecificityThe exercise program should be designed to load specific target bone(s) or body sites. For example, if gains in bone mass in the hip and femur are desired, an exercise plan that loads those specific sites, such as running or jumping, is indicated.

OverloadIn order to stimulate BMD gains in a particular bone, an exercise must overload that bone. This load imposed on a bone during exercise must be substantially greater than that experienced during normal activities of daily living. In other words, there is a certain threshold of loading which needs to be reached in order to produce a bone mass gain. However, much research is needed to more completely identify the recommended intensities for optimal bone adaptation.

ReversibilityIn adults, any gains made in BMD during an exercise program will likely be lost if the program is stopped. However, various studies with children and teenagers (whose skeletons are still growing) suggest some of the gains achieved from increased mechanical loading during an exercise program may be retained, even if the exercise program is discontinued.

Initial ValuesSubjects with below average initial BMD values will show greater gains in bone mass with exercise, as compared to persons with average or above average values.

Diminishing ReturnsThe greatest gains in BMD will be seen early on in an exercise program. After the initial increase, gains will continue, but at a slower pace. Knowledge of these principles when designing an exercise program will help personal trainers maximize their clients bone mass gains in critical regions of the body. As well, to prevent these gains from being lost, the concept of lifetime exercise needs to be wholly encouraged, especially for adults. It may be helpful to periodically remind adult clients that the BMD benefits that they acquire from exercising are only sustained with the maintenance of their exercise program.

EXERCISE STUDIES ACROSS THE LIFESPANA properly designed exercise program, which takes into account the five training principles discussed above, will increase bone strength in a wide variety of populations. Still, it is important to take into account the specific circumstances of each individual, such as age, hormonal status, and nutritional status, as these factors also play an important role in determining bone health.

EXERCISE AND BONE MASS IN ADULTS.Numerous studies have demonstrated that adults who exercise have greater bone mass than those who are less active. The degree of increase in bone density depends on the mode and intensity of exercise, the number of years training, and the age at which training began. Interestingly, higher BMD in athletes has been found at the body sites loaded during their respective sports, validating the principle of specificity elaborated above. As well, certain activities may not apply a sufficient enough overload to cause an increase in bone mass. Athletes who perform moderate to high-intensity impact activities, such as in gymnastics, power lifting, and jumping activities, have been found to have greater BMD than those performing non-impact or low-intensity activities, such as swimming and cycling. Some elite swimmers have even been found to have a lower BMD than non-exercisers, apparently due to unloading of their bones, which takes place during extending periods of time in a buoyant environment. It is especially beneficial for adults with low initial BMD values to begin exercise programs that enhance BMD, as these are the individuals who will see the most impressive gains.

EXERCISE AND BONE MASS IN CHILDRENResearch done on children and adolescents has confirmed a direct relationship between physical activity and increased bone mass. For example, studies in children have consistently found higher BMD values in dominant limbs when compared to non-dominant limbs. Other investigations have found that young athletes who load their dominant limbs preferentially while exercising (such as tennis players) have even greater disparities in BMD between their dominant and non-dominant sides. Additionally, jumping and other weight-bearing activities have been found to provide a significant increase in bone density at the spine and hip in prepubescent children. Some recent evidence suggests that exercise provides the best long-term benefits in BMD when it is initiated before puberty. As in adults, exercise interventions in children should be designed taking into account the principles of specificity, overload, reversibility, initial values, and diminishing returns. Unlike adults, children have been found to maintain their BMD gains from weight-bearing exercise, even if the exercise program is discontinued.

EXERCISE AND BONE MASS IN POSTMENOPAUSAL WOMENDue to the decline in circulating estrogen and resulting bone loss commonly seen in postmenopausal women, weight-bearing exercise to stimulate BMD and bone strength is especially important for this population cohort. Weight-bearing exercise and resistance training in post-menopausal women has been shown to increase and/or maintain BMD compared with losses observed in control groups. Research has also found that low-impact activities, such as walking, are not effective exercise interventions for preventing bone loss in post-menopausal women. Thus, the principle of overload should be applied, and exercises that provide a more substantial load on bone should be included. For instance, jogging and weight training may increase bone density in healthy post-menopausal women in a site-specific manner.

Studies of postmenopausal women incorporating exercise with hormone replacement therapy (HRT) currently show varied interactions with the prevention of bone loss. In some studies, exercise enhances the effect of HRT, whereas in other investigations no interaction has been observed combining HRT with exercise.

EXERCISE AND CALCIUM INTAKEThe effects of bone loading with adequate calcium intake are not fully identified and appear to vary during ones lifespan. However, calcium is a major mineral that is vital to the bones, and is needed for the heart, muscles and nerves to function properly for the blood to clot. General recommendations for intake are as follows: 1,300 mg/day for girls 9 to 18 yr, 1,000 mg for women 19 to 50 yr., and for 1,200 for women over 50 yr (Gottlieb, B., 2002).

FALLING AND FRACTUREThe ultimate goal of increased bone mass is to prevent osteoporosis-related fractures. Unfortunately, many studies have shown little or no reduction in this type of fracture between those who exercise and those who dont. It seems that while increased physical activity does stimulate gains in BMD, being more active also elevates the likelihood of falls, which often cause fractures. For this reason, any exercise program designed to increase BMD should also incorporate intervention strategies for balance, postural stability, muscular fitness and functional mobility to prevent future falls.

CONCLUSIONWhen designing an exercise program to increase bone mass, the principles of specificity, overload, reversibility, initial values, and diminishing returns should be emphasized. An exercise regimen can increase BMD at a specific body site, but only if the load imposed by exercise is far greater than that encountered in normal activities of daily living. Since the ultimate goal of increasing bone mass is to prevent osteoporosis-related fractures, a well-designed exercise program should also address the issues of balance, coordination, fall prevention, and functional strength. Ideally, weight-bearing activities such as running and jumping should begin at an early age and continue throughout life, or at least until they are no longer considered safe and feasible. Finally, to fully assist your clients with a lifestyle osteoporosis prevention approach, share with your clients the recommendations from the National Osteoporosis Foundation (See Table 1).